Recently, development of portable electronic machinery and tools has been accelerated by practical use of a lithium secondary battery that provides high voltage and high capacity, and as a result demand for the lithium secondary battery has been increasing. The lithium secondary battery that provides high voltage and high capacity has been materialized by using a material that is capable of introducing and releasing lithium, for the negative electrode, and using a complex oxide comprising lithium and a transition metal, for the positive electrode. However, improvement has still been desired in the technical point of charge/discharge cycle life.
Hitherto, attempts to improve cycle characteristics have been made in various fields, in particular regarding the solvent composition of the electrolytic solution and kinds of a supporting electrolyte, because an electrolytic solution-related reaction is involved during a charge/discharge cycle.
Further, an attempt to prevent an electrolytic solution from decomposition by containing a small amount of additives in an electrolytic solution, whereby the cycle life is prolonged, has been made. For example, JP-A-59-3874 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-63-269461, and JP-A-8-321313 describe that boric acid esters, such as trimethyl borate, are added to an electrolytic solution. Further, JP-A-3-236169 describes that silylated boric acid esters are added to an electrolytic solution.
However, the above-mentioned techniques have not yet reached the level that a high discharge capacity is compatible with excellent cycle characteristics.